Final answer:
The central ray that passes through the center of a thin lens does not change direction and is undeviated, a characteristic important for practical applications like light collimation in lighthouses and traffic lights.
Step-by-step explanation:
The ray passing through a thin lens that does not change direction is the ray that passes through the center of the lens. In ray tracing for thin lenses, which involves following the paths that light rays take, there is a specific rule that applies to such lenses. The rule states that a ray which passes through the center of a thin lens is deflected by a negligible amount and is therefore assumed to emerge parallel to its original path. This characteristic allows for simpler ray tracing, especially important when dealing with the basic functioning of lenses in various applications such as forming images on film, magnifying text, or correcting vision problems.
The principle behind this is that a thin lens is defined to be of negligible thickness compared to the radii of curvature of its surfaces, allowing light rays to be considered as bending only once at the lens's center. Therefore, practical applications such as lighthouses or traffic lights may make use of this property to produce directional beams of light. In summary, the particular ray that does not change direction when it passes through a thin lens is the central ray, known for its undeviated path.